This relates generally to three-dimensional (“3D”) display systems, and more particularly to optical system designs for generation of light fields using spatial light modulators (“SLMs”).
Some conventional visual display units (including computer screens, television screens and movie theater screens) produce a two-dimensional image that lacks true depth perception. Past efforts have been made to create visual displays for producing images having three dimensions (“3D”). Some conventional 3D displays produce a 3D effect by presenting each eye with a different scene, typically shifted in position to generate a parallax effect. In a simple example of these 3D displays, 3D glasses provide a first view to a first eye and a second view to a second eye. One such example is disclosed in U.S. Pat. No. 5,113,285 to Franklin, et al. These 3D glasses use either: a state of light polarization to separate the images; or a high speed shutter over each eye to isolate the first eye view from the second.
Some 3D displays generate imagery using a multiview architecture, instead of requiring special glasses. These systems create a different view of the scene for each position of the observer, but the observer is often limited to a small viewing area, or eyebox. One example of such a multiview display system is disclosed in U.S. Pat. No. 6,798,409 to Thomas, et al.
Some 3D displays use the concept of “light fields” to avoid a requirement for special glasses, and to avoid confining the viewer to a fixed or small set of viewing positions. These light field displays create an image similar to a hologram's image, but these displays generate a dynamic image without using photographic recording holographic plates or films.
A light field includes rays of light that pass through a viewing window. Those rays are emitted from objects either beyond or in front of the viewing window, so all of the light rays reflected from objects viewed through the viewing window are captured at a particular instant in time and space. Such a light field is shown schematically in FIG. 1, where a number of light rays (including those designated 30 and 40, reflecting off of different objects in space) pass through a common plane or viewing window 20. Each such light ray has a particular x-y position and a particular angular direction as it passes through the viewing window 20.
The general concept of light fields is discussed in greater detail within the technical paper entitled “Light Field Rendering,” by Marc Levoy and Pat Hanrahan of the Computer Science Department at Stanford University, Proc. ACM SIGGRAPH, 1996. The viewing of the light field is the most natural way of viewing objects. Light rays emitted by the light field contain information about the position, color, brightness and extent of the viewed objects. Artificially reproducing this light field will create a plane emitting light rays that, when observed, will appear to reproduce a scene in full 3D, as if the viewer is seeing real objects emitting such light.
To create an artificial light field, some display systems use many spatial light modulators (“SLMs”) to satisfy high pixel density requirements of high-quality 3D image representation. An SLM device is capable of receiving a beam of light, receiving electrical signals defining a desired graphic pattern, and selectively modulating the received light beam (relative to amplitude, phase and/or polarization of its light waves in space and time) to produce a patterned two-dimensional light image incorporating the desired graphic pattern.
Two types of SLMs are translucent and reflective. A commonly used modulation mechanisms is the electro-optical spatial light modulator containing liquid crystals as the modulation material. The optical properties of the liquid crystals are modified by an electric field. The translucent SLMs typically use liquid crystal display (LCD) elements. The incoming light pattern is directed at the front face of the LCD display, and the modulated light pattern passes out through the rear face of the LCD display. Some reflective SLMs use liquid crystal on silicon (“LCoS”) display elements to modulate an incoming light beam through selective reflection of rays of light. The SLM includes one or more control inputs for receiving control signals, typically from a computer, specifying the graphic pattern to be produced by the SLM.
Another form of SLM is a digital micromirror device (“DMD”), commercially available from Texas Instruments Incorporated of Dallas, Tex. A DMD includes an array of highly reflective aluminum micromirrors. The DMD is an electrical input, optical output micro-electrical-mechanical system (“MEMS”) that performs high speed, efficient and reliable spatial light modulation. The DMD is manufactured using semiconductor manufacturing processes. Each DMD contains up to 8 million individually controlled micromirrors built over an associated CMOS memory cell. During operation, the DMD controller loads each underlying memory cell with a logical “1” (“on”) or a logical “0” (“off”). Next, a mirror reset pulse is applied, which causes each micromirror to be electrostatically deflected about a hinge to either a +12° state or a −12° state, depending upon whether the underlying memory cell is a “1” or a “0”. The array of micromirrors reflect incoming light at different angles, depending upon the binary logical state of each micromirror. When viewed perpendicular to the reflective surfaces of the “on” micromirrors, a desired patterned image is observed. In a projection system, the +12° state corresponds to an “on” pixel, and the −12° state corresponds to an “off” pixel. Grayscale patterns are created by programming the on/off duty cycle of each mirror, and multiple light sources can be multiplexed to create full RGB color images.
As mentioned hereinabove, 3D display systems can use the concept of light fields. To create the light field, a relatively large number of SLMs can produce a corresponding number of images that are “tiled” together to form a composite larger image. To create the light field, each SLM provides a bundle of addressable, modulated light rays, which are focused through a small region. This small region is called a light “hogel.” An array of such hogels, each having its own set of addressable angular distribution of light rays, forms the basis of the light field display.
In one example, a high quality light field display has: a high hogel density within a two-dimensional plane; and, from each hogel, a high density of addressable light rays, each having specific angular orientation. Also, a 3D dynamic image requires updating these hogels at a relatively high refresh rate. These conditions create a need for displays with a very high pixel density, including with a very large number of pixels that can be updated at high speed to generate the array of hogels and their far field ray distributions, as required to create a dynamic 3D light field display. Unfortunately, such displays have required a large number of relatively expensive SLMs to create this large number of pixels.
Further, conventional 3D display systems often require high intensity light sources to provide light to illuminate SLMs, which can add to the physical size, power requirements and cooling requirements of 3D projection systems.